The present invention relates to the alleviation of hypotension induced by activators of nitric oxide production, particularly as related to the restoration of sensitivity to known pressor agents such as .alpha..sub.1 adrenergic agonists.
It is a well known clinical observation that septic or cytokine-induced shock patients are insensitive to clinically used pressor agents. The basis for this insensitivity was not understood until now. The present invention is a means to restore pressor sensitivity in this clinical class of patients, based on the discovery that nitric oxide overproduction is the basis for pressor insensitivity.
In 1980, Furchgott and Zawadski (Nature 288: 373-376) demonstrated that endothelial cells, which line blood vessels, can be stimulated to release a substance which relaxes vascular smooth muscle i.e., causes vasodilatation. Since the chemical nature of this substance was completely unknown, it was simply named endothelium-derived relaxing factor (EDRF). It is now widely accepted that many naturally-occurring substances which act as physiological vasodilators mediate all or part of their action by stimulating release of EDRF; these substances include, acetylcholine, histamine, bradykinin, leukotrienes, ADP, ATF, substance P, serotonin, thrombin and others. Although the extremely short lifetime of EDRF (several seconds) hampered efforts to chemically identify this molecule, in 1987 several laboratories suggested that EDRF may be nitric oxide (NO), which spontaneously decomposes to nitrate and nitrite. A fundamental problem in accepting this NO hypothesis was that mammalian systems were not known to contain an enzymatic pathway which could synthesize NO; additionally, a likely precursor for NO biosynthesis was unknown. After observing that the arginine analog L-N.sup.G -methylarginine (L-NMA) could inhibit vascular EDRF/NO synthesis induced by acetylcholine and histamine, and that EDRF/NO synthesis could be restored by adding excess L-arginine, it was proposed that arginine is the physiological precursor of EDRF/NO biosynthesis (Sakuma et al., PNAS 85: 8664-8667, 1988). Additional evidence supporting this proposal was reported almost simultaneously. It was later demonstrated that inhibition of EDRF/NO synthesis in the anesthetized guinea pig raises blood pressure, suggesting that EDRF/NO is an important physiological regulator of blood pressure (Aisaka et al., BBRC 160: 881-886, 1989). Notwithstanding the accumulated evidence supporting a role for NO in vascular homeostasis, it is understood by those skilled in the art that other nitrogen oxides may be present and may be active in reducing blood pressure. Within this specification, the acronym NO will be understood to represent nitric oxide and any additional vasoactive nitrogen oxides.
Other laboratories had demonstrated that macrophage cells become "activated" by 12-36 hour treatment with gamma-interferon, bacterial endotoxin and various cytokines. This "activation" is associated with initiation of tumor cell killing and generation of nitrite and nitrate from L-arginine. It was observed that activated macrophages actually make NO from L-arginine (just like endothelial cells) and that this NO subsequently reacts with oxygen to form more oxidized nitrogen metabolites which appear to be physiologically inert (Stuehr et al., J. Exp. Med. 169: 1011-1020, 1989). The enzyme responsible for NO synthesis (nitric oxide synthetase) has been partially characterized (Stuehr et al. BBRC161: 420-426, 1989) and acts to oxidize the terminal amino group of arginine, resulting in production of NO and citrulline. It is now believed that macrophage-derived NO is an important tumoricidal and bactericidal agent. Since bacterial endotoxin, gamma-interferon and other cytokines can trigger NO generation by macrophage cells (which are not known to play a role in vasoregulation) it appeared possible that: 1) NO generation may also be stimulated by similar stimuli in other cell types in the vessel wall that do impact on vascular tone and 2) septic shock (i.e., systemic vasodilatation induced by bacterial endotoxin) may result from massive activation of NO biosynthesis. Speculation that the latter hypothesis was correct was fueled by a prior report that urinary nitrate levels are grossly elevated by treatment of rats with bacterial endotoxin (Wagner et al., PNAS 80: 4518-4521, 1983).
Cytokines are well known to cause morphological and functional alterations in endothelial cells described as "endothelial cell activation". Distinct immune-mediators such as tumor necrosis factor (TNF), interleukin-1 (IL-1), and gamma-interferon (IFN or I) appear to induce different but partially overlapping patterns of endothelial cell activation including increased procoagulant activity (Bevilaqua, 1986), PGI2 production (Rossi, 1985 Science 229,174), HLA antigen expression (Pober 1987) and lymphocyte adhesion molecules (Harlan 1985; Cavender 1987). Although these cytokines are reported to cause hypotension, vascular hemorrhage, and ischemia, the underlying mechanisms of altered vasoactivity are unclear (Goldblum et al. 1989; Tracey et al. Science 234:470, 1986). A potential mediator of altered vasoactivity is EDRF.
In both clinical and animal (Dvorak, 1959) studies on the effects of biological response modifiers, a major dose limiting toxicity has been hypotension and vascular leakage.
The use of arginine-based inhibitors of nitric oxide synthesis for the treatment of endotoxin(sepsis)- and cytokine-induced hypotension relates to the discovery that nitric oxide, a potent vasodilator, is overproduced in these conditions.
While pressor drug therapy (i.e., adrenergic agonists) is a commonly employed method for attempting to restore blood pressure in septic patients, pressor agents are typically ineffective in these patients (a significant fraction subsequently dying). Indeed, insensitivity to vasoconstrictors is a characteristic of septic shock and considered to be a major impediment to effective pharmacotherapy. The mechanistic basis for this insensitivity of septic blood vessels of vasoconstrictors is incompletely understood.
On the other hand, it has been known for many years that nitric oxide dilates isolated blood vessels which had been previously constricted with various pressor agents. Thus NO is known to reverse the action of vasoconstrictors in vitro. More recently, the present inventors have found that endotoxin and cytokines induce the synthesis of large quantities of nitric oxide in the blood vessel wall. Taken together, these observations suggested the possibility that excess nitric oxide synthesis may be the cause of pressor drug efficacy in septic patients. The findings presented herein support this view and indicate that inhibition of arginine-derived nitric oxide synthesis will, in addition to restoring blood pressure, restore pressor sensitivity in septic patients.